T-4

Environmental Applications of Non-thermal Plasmas
非平衡プラズマを用いた環境応用技術
Akira Mizuno
水野彰

Department Ecological Engineering, Toyohashi University of Technology,Tempaku-cho, Toyohashi, 441-8580, Japan
豊橋技術科学大学 エコロジー工学専攻, 〒44-8580 愛知県豊橋市天伯町雲雀ヶ丘 1-1

Non-thermal discharge-plasma is effective for abatement of gaseous pollutants, and commercial
products have been introduced recently. Indoor air cleaners have been mass-produced.
Non-thermal plasma is also effective to decompose volatile organic compound (VOC) or
cleaning of diesel exhaust. Combination of plasma and catalysts will further improve efficiencyand selectivity. Generation of non-thermal discharge plasma, its application in gas cleaning willbe presented.
very important for improvement of energy
efficiency and selectivity of plasma chemical
processes.
In this paper, several commercialized
applications are introduced, together with possible
applications such as diesel exhaust cleaning, etc.

1Introduction
Non-thermal discharge plasma in atmospheric
air produces reactive radicals (O, OH, etc.) that
promote chemical reactions [1-2]. Non-thermal
plasma has been applied for removal ofodor/allergen in indoor air, VOCs in factories, etc.
These applications require high energy efficiency,
low cost and reduced size. To improve the energy
efficiency, combination of non-thermal plasma withvarious catalysts has been investigated, and has
been applied in several commercialized products forindoor air cleaning and VOC decomposition.
DeNOx of diesel exhaust can be made, and ispotentially an important application.
There areseveral types of non-thermal discharge to be used
for gas cleaning, such as pulsed streamer
discharge[3], packed-bed[6], etc. In air, or flue gas,
O and OH radicals are produced at first, then theseradicals react with gaseous pollutants. Chemical
reactions on catalyst surface exposed to plasma areFront view
6300 mm

H.V.

50 m mH.V.

Gas
TiO coated aluminum mesh2(a) Electrode and catalyst
Fig.1

Applied Voltage [kV]Side view
12 mm

2Indoor air cleaning using discharge plasma
with titanium dioxide catalyst
As a pioneering commercial product, TiO2
catalyst was used, combined with positive streamer
discharge[3] generated by dc with ac superposition.
Fig.1 shows the reactor, consisting of the parallel
plate with the wire discharge electrode, follolwed
by a TiO2 catalyst. The gas residence time was10msec. One pass performance is about 70 % fordust removal, and 27 % for 1 ppm acetaldehyde
removal. TiO2 can be activated by photons having
more than 3.2eV. In this reactor, the TiO2 catalyst
may possibly be activated by energy exchange ofhigh energy particles produced by the plasma[3].

420020406080
Time [ µsec](b) Voltage waveform

Indoor air cleaner using pulsed discharge with TiO2 catalyst

1001203Cleaning of diesel exhaust
Cleaning of diesel engine exhaust is potentiallyan important application of non-thermal plasma.
Reduction of NOx to N2 and O2 is the most
attractive way, especially for vehicles. The
combination of discharge plasma with catalysts or
chemical scrubbers has been emerged as a potential
method [4-5]. Fig.2(a) shows the 2-stage plasma
process. NO is oxidized to NO2 and entering to theccatalyst, where low temperature reduction of NO2
can be made by adding reductive agent such asammonia. Fig. 2(b) is an example of the NOxremoval in the two-stage system with Co-ZSM-5
catalyst at 150 C[5]. NO was completely oxidized
to NO2 with 27kJ/Nm3 of specific input energy inthe plasma reactor, and introduced to the catalyst
section.
To assist the oxidation of NO to NO2 in thedischarge plasma reactor, 1200ppm of ethylene wasadded. At the inlet of the catalyst section, 200 or
400 ppm of ammonia was added. At 150 C,
continuous reduction of NO2 can be made. No
powder formation was observed in this operation.
Carbon soot could be used as a reductant agent for
NOx when the soot layer absorbing NO2 is exposed
to discharge plasma[6].
4Concluding remarks
Non-thermal discharge plasma has been utilized forcleaning of indoor air and flue gas. In order toimprove the energy efficiency, appropriate
combination with catalysts is effective. The
mechanism, however, has not been clarified.
Further understanding of surface, gas and liquid

phase plasma chemical reactions will provide more
effective means to improve the efficiency, and morewide industrial applications such as sterilization,
chemical synthesis, surface modification, thin film
deposition at atmospheric pressure, water
purification, etc.
References[1] Non-thermal plasma Techniques for PollutionControl” NATO ASI Series, Vol. 34, Part A andPart B, Edited by B.M. Penetrante and S.E.
Schultheis, Springer-Verlag, 1993.
[2] Hackam and Akiyama, “Non-thermal plasma
technology for gaseous pollution control”, IEEE
Trans. on Dielectrics and Electrical Insulation, 7,
pp. 615-624, 2000.
[3] A. Mizuno, Y. Kisanuki, M. Noguchi, S.
Katsura, S.H. Lee, Y.K. Hong, S.Y. Shin, andJ.H. Kang, “Indoor Air Cleaning using a Pulsed
Discharge Plasma”, IEEE Transactions onIndustry
Applications,Vol.35,
No.6,pp.1284-1288, 1999.
[4] B.M. Penetrante, R.M. Brusasco, B.T. Merritt,W.J. Pitz and G.E. Vogtlin, M.C. Kung, H.H.
Kung, C.Z. Wan and K.E. Voss, Proc. of SAE
meeting, pp.57-66, 1998.
[5]H.H. Kim, K. Takashima, S. Katsura and A.
Mizuno, J. Phys. D: Appl. Phys. 34, p.604-613,
2001[6] Yoshihiko Matsui, Satoshi Sato, Kazunori
Takashima, Shinji Katsura and Akira Mizuno,
SAE Technical paper, No.2003-01-1185(SP-1759), pp.111-119, 2003

200 ppm（400ppm) 80mm

200mm
80mm

Discharge Reactor
80m

4.0L/minNO(400ppm)
O2(10％)
N2Honeycom Catalyst (Co-ZSM5-Zeolite)
The number of cell : 225 , Cross-section : 9mm2
Gas Hourly Space velocity : 2000[h-1](a)2-stage reactor

400 ppm

400
Concentration (ppm)

C2H4(1200ppm)

NH３NONOx

300

200

100001234Time (hr)5(b) NO removal performanceFig.2 NOx removal in the two stage process with Co-ZSM-5 catalyst.
(SIE: 27kJ/Nm3, GHSV: 1820hr-1, Temperature: 150 C, C2H4: 1200ppm,6